Recently, there has been a growing demand for magnetic recording mediums such as magnetic recording tapes that are capable of reproducing pictures and sounds of higher quality. To satisfy such demand, further improvement in electromagnetic conversion characteristics, light-shielding property and erasing property (a property of not changing its electromagnetic conversion characteristics with the passage of time) must be attained. For this purpose, a great deal of efforts have been made in the art.
As part of these efforts, Japanese Patent Publication Open to Public Inspection (hereinafter abbreviated as Japanese Patent O.P.I. Publication) No. 61230/1986 proposes the use of a larger amount of Fe.sup.2+ in a magnetic layer, aiming at improving light-shielding property and erasing property, as well as increasing output. However, when contained in a magnetic layer in a larger amount, Fe.sup.2+ due to its poor dispersibility, tends to make the surface of the magnetic layer rough and cause dropouts to lower the electromagnetic conversion characteristics of a recording medium. Therefore, the use of an excessive amount of Fe.sup.2+ should be avoided.
On the other hand, the use of Fe.sup.2+ in a smaller amount was proposed as a measure to prevent the surface of a magnetic layer from getting rough. In this case, lowering in light-shielding property which may occur due to the scarcity of Fe.sup.2+ has to be avoided by the employment of carbon black. However, the use of carbon black inevitably results in the reduction of the content of magnetic powder in a magnetic layer, which then leads to the deterioration of electromagnetic conversion characteristics.
The surface of a magnetic layer can be prevented from getting rough by the addition of a lubricant (fatty acid).
Proposals that were made as to the effective use of a lubricant include: (a) employing a lubricant in such a manner that the average lubricant content of a lower magnetic layer will be 3/4 or 3/2 of that of an upper magnetic layer (see Japanese Patent O.P.I. Publication No. 264509/1986); (b) employing a lubricant only in a lower magnetic layer (see Japanese Patent O.P.I. Publication No. 200426/1983); and (c) employing a lubricant in such a manner that the average lubricant content of a lower magnetic layer will be 1.2 times higher than that of an upper magnetic layer (see Japanese Patent O.P.I. Publication No. 264509/1986).
These efforts, though they were made to improve the running durability of a magnetic recording medium, could not be rewarded with good fruits; by the method (b), a recording medium can be kept in close contact with a magnetic head, but its durability against repeated running cannot be improved to a sufficient level. In the method (c), a recording medium cannot be in close contact with a head, and, in addition, there may occur such problems as lowered running durability due to the plasticization of a magnetic layer, and sticking of dust to a magnetic head at high temperatures.
When employed in combination with a certain kind of magnetic substance, a lubricant tends to cause such problems as the blooming of a magnetic layer surface and the staining of a magnetic head. Taking such side effects of a lubricant into consideration, it may be concluded that the characteristics of a magnetic recording medium cannot be improved by simply adding a lubricant.